Fix reversed args in mount call

Riya Khanna reported that with a ramfs rootfs the mount to make
/ rprivate was returning -EFAULT.  NULL was being passed as the
mount target.  Pass "/" instead.

Reported-by: riya khanna <riyakhanna1983@gmail.com>>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>

src/lxc/conf.c

@@ -1513,7 +1513,7 @@ int prepare_ramfs_root(char *root)
 		return -1;
 	}
 
-	if (mount(".", NULL, NULL, MS_REC | MS_PRIVATE, NULL)) {
+	if (mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL)) {
 		SYSERROR("Failed to make . rprivate");
 		return -1;
 	}